DC-DC converters are commonly used in integrated circuits for providing stable voltages. There are many varieties of DC-DC converters. FIG. 1 illustrates a circuit diagram of a conventional on-chip current sensing circuit in a current programmed mode (CPM) DC-DC converter.
In the current sensing circuit shown in FIG. 1, current IL″, which is also the charging current during the on state of the DC-DC converter, is sensed. Transistor M1″, when turned on, may conduct inductor current IL″ to inductor L″ and capacitor CL″. Inductor current IL″ flows through sense resistor Rsense″ to generate a voltage between the positive input and the negative input of operational amplifier OP. The output voltage at node OPout thus reflects the voltage on sense resistor Rsense″, and reflects inductor current IL″. The voltage at node OPout and a saw-tooth voltage is summed by a summation circuit (not shown) and fed to a control logic generator, which comprises comparator U1″, flip-flop U2″, and pre-driver U3″. The control logic generator generates signals for controlling the operation of transistor M1″.
It is realized that inductor current IL″ has a relatively great amplitude. On the other hand, to maintain the accuracy and to reduce the process variation of sense resistor Rsense″, sense resistor Rsense″ cannot be too small. This means that a considerable amount of power is wasted by sense resistor Rsense″, especially in high-load applications.
FIG. 2 illustrates the circuit diagram of another conventional current sensing circuit, which includes transistors M1′ and M2′ that may form a current mirror during the ON state of the respective DC-DC converter. With transistor M1′ having a greater aspect ratio (a ratio of gate width to gate length) than transistor M2′, the inductor current IL′ that flows through transistor M1′ is mirrored to a smaller current IM2′ that flows through transistor M2′. The sense current flows through resistor Rsense′ to generate a sense voltage at node C′. The value of sensing current Isense′ equals to the difference between current IM2′ and current I2 of a constant current source. Switches 51 and S2, comparator U1′, flip-flop U2′, and pre-driver U3′ are used to control the operation of the control logic generator.
If the ratio of the aspect ratio of transistor M1′ to the aspect ratio of transistor M2′ is N, and the voltage at node A′ exactly matches that of node B, the 12V ratio of inductor current to the sensing voltage at node C (during the on-state of the DC-DC converter) may be expressed as:
                                                                        I                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                V                            =                                                IL                  ′                                                                      (                                                                                            IL                          ′                                                /                        N                                            -                                              I                        ⁢                                                                                                  ⁢                        2                                                              )                                    ×                                      Rsense                    ′                                                                                                                          =                                                1                                                                                    Rsense                        ′                                            /                      N                                        -                                          I                      ⁢                                                                                          ⁢                      2                      ×                                                                        Rsense                          ′                                                /                                                  IL                          ′                                                                                                                    ≈                                  N                  /                                      Rsense                    ′                                                                                                          [                  Eq          .                                          ⁢          1                ]            
It is observed that the element I2×Rsense′/IL′ may be neglected if it is much smaller than Rsense′/N. However, it may adversely result in the non-linearity in the 12V ratio if inductor current IL′ is small enough to cause I2×Rsense′/IL′ to be comparable with Rsense′/IL.
Further, bipolar transistors Q1 and Q2 are used to maintain equal voltages at nodes A′ and B′, so that currents IL′ and IM2′ may be proportional to each other accurately. However, the voltage mirror that employs bipolar transistors Q1 and Q2 is not capable of forcing an accurate matching of the voltages. As a result, the accuracy of the current sensing circuit is seriously degraded.
A further drawback of the current sensing circuit in FIG. 2 is the demanding requirement to supply voltage VIN. The voltage at node A′ needs to be great enough in order to turn on NMOS transistor M3′ and PNP transistor Q2, so that inductor current IL′ may be sensed as intended. Accordingly, voltage VA′ at node A′ needs to satisfy the requirement:VA′≧VC′+VTH(M3′)+VEB(Q2)  [Eq. 2]
Wherein voltage VC′ is the voltage at node C′, voltage VTH(M3′) is the threshold voltage of transistor M3, and voltage VEB(Q2) is the emitter-to-base voltage of bipolar transistor Q2. Thus, if power supply voltage VCC of the DC-DC converter is less than 2V, the demanding requirement to voltage VA′ at node A′ cannot be satisfied, and the current sensing circuit cannot work correctly.